1. Field of Invention
This invention relates to a surface light source device of side light type, and more particularly, to a surface light source device of side light type, which shows improved uniformity in luminance. The present invention is applied to backlighting in a liquid crystal display, for instance.
2. Related Art
It is a matter of common knowledge that a surface light source device of side light type provides a thin backlighting arrangement to illuminate a liquid crystal display panel. In general, the surface light source device of side light type has a light guide plate made of a plate-like light guiding member and a primary light source including a long cylindrical light source, and the primary light source is arranged on the lateral side of the light guide plate.
Illumination light emitted from the primary light source is introduced into the light guide plate through an end surface of the light guide plate. The introduced illumination light is refracted and then emitted toward a liquid crystal display panel through one of the major surfaces of the light guide plate. Since the primary light source is arranged on the lateral side of the light guide plate, it is possible to easily provide a device of thin type on the whole.
A well-known light guide plate employed in the surface light source device of side light type as described above is classified into a light guide plate of a type which is substantially uniform in thickness, and a light guide plate of a type which shows a tendency to decrease a thickness according as the light guide plate becomes more distant from the primary light source. Generally, the light guide plate of the latter type emits illumination light more efficiently than that of the former type.
FIG. 5 is an exploded perspective view showing a surface light source device of side light type employing the light guide plate of the latter type. FIG. 6 is a sectional view taken along a line A--A in FIG. 5. Referring to FIGS. 5 and 6, a surface light source device of side light type 1 has a light guide plate 2, and a primary light source 3 is disposed on the lateral side of the light guide plate. The surface light source device 1 further has a reflection sheet 4, a diffusible sheet 5, prism sheets 6, 7 function as a light control member, and a protective sheet 8 of low diffusibility, and these components are laminatedly arranged together with the light guide plate 2 as shown in FIG. 6.
The primary light source 3 has a cold cathode tube (a fluorescent lamp) 9 and a reflector 10 surrounding the cold cathode tube. The reflector 10 has an aperture, through which illumination light is supplied to an end surface 2A of the light guide plate 2. The reflector 10 is made of a regular reflective or diffuse reflective sheet material or the like. The light guide plate 2 having a wedge-shaped section is made of an acrylic material (PMMA resin), for instance, by means of injection molding. The light guide plate 2 receives the illumination light from the primary light source 3 through one end surface defined as an incidence surface 2A.
In the light guide plate 2, the illumination light makes a propagation while undergoing repetitive reflection between a major surface (which will be hereinafter referred to as "a slope") 2B, along which the reflection sheet 4 is disposed, and a different major surface (which will be hereinafter referred to as "an emitting surface") 2C, along which the diffusible sheet 5 is disposed.
Every time reflection occurs, a component of light incident at an angle of not more than a critical angle is emitted through the slope 2B and the emitting surface 2C. Accordingly, the illumination light emitted through the emitting surface 2C is inclined to mainly make a propagation toward the wedge end. This phenomenon is called emitting directivity.
Further, a diffusible surface 2D is formed on the slope 2B. The diffusible surface 2D is provided with diffusibility which steps up from the side of the incidence surface 2A toward the wedge end. The diffusibility is provided by means of coating the slope with diffusible ink containing a pigment consisting of magnesium carbonate, titanium oxide or the like, for instance. The degree of diffusibility may be adjusted according to a quantity of diffusible ink coated on the slope.
The slope 2B may also be provided with diffusibility by means of matting (roughening) the slope 2B, instead of coating the slope with the diffusible ink. In this case as well, such diffusibility as sets up from the side of the incidence surface 2A toward the wedge end is provided. Thus, matted surface areas of a rectangular shape, for instance, are formed in a distributed state at a certain or random pitch such that the density of distributed areas sets up from the side of the incidence surface 2A toward the wedge end.
The light guide plate 2 as described above corrects a quantity of output light decreased in the vicinity of the wedge end, and makes the distribution in quantity of output light uniform. It is to be noted that such diffusibility is not so intensive as the light guide plate is allowed to lose the emitting directivity. That is, even if the diffusibility is provided to the slope 2B, the illumination light emitted through the emitting surface 2C is inclined to mainly make a propagation toward the wedge end.
The reflection sheet 4 is made of a sheet-like regular reflective member consisting of metal foil or the like or a sheet-like diffuse reflective member consisting of a white PET film or the like. Illumination light leaking out of the slope 2B is incident again onto the light guide plate 2 after having been reflected by the reflection sheet 4, resulting in improvement of efficiency of illumination light utilization.
The prism sheets 6, 7 are arranged to correct the emitting directivity of the light guide plate 2. The diffusible sheet 5 is arranged to prevent the diffusible surface 2D on the slope 2B from being visibly observed from above the emitting surface 2C, and also to make highlight and shadow or the like in each part of the light guide plate 2 illuminated with the illumination light less noticeable.
The diffusible sheet 5 diffuses the illumination light emitted through the light guide plate 2. The prism sheets 6, 7 are made of a light-transmitting sheet material such as polycarbonate. In each prism sheet, a surface (an outside surface) with its back to the light guide plate 2 is formed as a prism surface. The prism surface is composed of a large number of projections which are respectively triangular in sectional shape and run approximately parallel in one direction. In the shown case, the projections on the inside prism sheet 6 are oriented so as to run parallel to the incidence surface 2A, while the projections on the prism sheet 7 are oriented so as to run in a direction orthogonal to the incidence surface 2A.
The prism sheets 6, 7 correct a main emitting direction of output light with a slope of each projection so as to emit the output light in a frontal direction of the emitting surface 2C. It may be also possible to use a so-called double-faced prism sheet having both surfaces respectively serving as prism surfaces.
In general, the surface light source device of side light type employing the wedge-shaped light guide plate and the prism sheets as described above may emit the output light in the frontal direction more efficiently than a surface light source device of side light type employing a light guide plate which is substantially uniform in thickness.
The protection sheet 8 protects the surface of the prism sheet 7 from damage or the like, and relieves the directivity of output light corrected by the prism sheets 6, 7 so as to enlarge an angle of visual field. Accordingly, the illumination light may be emitted to a desired extent in the frontal direction of the emitting surface.
FIG. 7 shows a surface light source device of side light type in another prior art. The surface light source device shown in FIG. 7 has a structure similar to that in the surface light source device shown in FIGS. 5 and 6, except that a reflection sheet 4 in the surface light source device shown in FIG. 7 extends to the vicinity of the incidence surface 2A of the light guide plate.